Chromogenic materials are characterized in that their optical properties may be changed in response of an external stimulus. The main chromogenic technologies are electrochromic (depending on electrical voltage or charge), thermochromic (depending on temperature), photochromic (depending on ultraviolet irradiation), and gasochromic (depending on exposure to reducing or oxidizing gases). The present invention relates to thermochromic materials. Chromogenic materials have been used e.g. in various optical applications, such as windows, mirrors, spectacles, visors etc. Different techniques have been selected, depending on the specific requirements in the different applications.
Thermochromic materials exhibit a reduction in transparency when the temperature increases. This reduction takes place mainly in the infrared region. Typically, there is a certain temperature around which the transparency in the infrared wavelength range is changed from a “light state” (high transparency) to a “dark state” (low transparency). For wavelengths in the visible region, the transparency is typically influenced very little. Similarly, the thermochromic materials switch from a dark state to a light state when the temperature goes under a certain critical temperature. This temperature is referred to as the transition temperature (upon cooling). Typically, the transition temperature upon heating is slightly higher than the transition temperature upon cooling due to hysteresis effects. Such hysteresis effects are probably at least to a part dependent on crystallinity imperfections in the films. However, for practical use, an average transition temperature is typically the most important to consider. Most of the best thermochromic surface coatings developed so far are based on vanadium dioxide (VO2). The material is structurally changed at a critical temperature, i.e. the transition temperature, which for pure VO2 is situated at about 68° C. The transmittance is increased when the temperature goes below the transition temperature. The use of pure VO2 thermochromic materials is in prior art limited by e.g. too high transition temperature and too low transparency in the visible region.
In prior art, there are many attempts to reduce the transition temperature by doping with different elements; a typical example is doping with W, c.f. e.g. the sections regarding thermochromic materials in “Transparent conductors as solar energy materials: A panoramic review”, by C. G. Granqvist, in Solar Energy Materials &. Solar Cells 91 (2007), sect. 5.2-5.3, pp. 1555-1558. Furthermore, in “Optical and electrical properties of vanadium oxides synthesized from alkoxides”, by J. Livage, Coordination Chemistry Reviews 190-192 (1999), pp. 391-403, a number of dopants—W6+, Nb5+, Ti4 and Al3+—have been tested as to their influence on the transition temperature. Tungsten could be used to lower the transition temperature, while doping with aluminium instead increases the transition temperature. Also, in the published international patent application WO01/14498, different transition metals of a valence in their oxides of at least 5 are used as dopants for lowering the transition temperature. Dopants selected from transition metals with a valence equal or below 4 may be selected if a higher transition temperature is required.
However, a remaining problem for being attractive in e.g. window applications is the low transparency of the thermochromic material for wavelengths in the visible range. A low transparency for these wavelengths gives an impression of a dark window and reduces the ability to watch details through the window.